Species‐dependent patterns of fructan synthesis by enzymes from excised leaves of oat, wheat, barley and timothy

Abstract

Excised leaves of oat, wheat, barley and timothy were illuminated continuously for 24 h and shown to accumulate both sucrose and fructan. The fructan complements of the four species were compared with the well‐characterized pattern from leaves of L. temulentum L. by thin‐layer chromatography (TLC). Wheat and barley had similar patterns of oligosaccharides, accumulating isokestose, kestose and a range of oligofructans up to an apparent degree of polymerization (DP) of 10. Oat leaves accumulated neokestose in addition to kestose and isokestose. The oligofructans of oat also extended up to DP 10, though the pattern was more complex than that of wheat and barley. Timothy leaves accumulated fructan predominantly of apparent DP ≤ 16 and little smaller oligofructan. Analysis of concentrated carbohydrate extracts showed isokestose, kestose and a ladder of oligofructans of apparent DP in the range 4 to ≤ 20 in induced timothy leaves. The results showed that leaves of each grass accumulated a distinct and species‐specific pattern of fructan. Concentrated enzyme preparations from illuminated leaves of each species were incubated at 30 °C with 400 mol m−3 sucrose at pH 4.6 in the presence of 16 mol m−3 pyridoxal hydrochloride. The preparations catalyzed the de novo net synthesis of trisaccharides and larger oligofructans at rates approximating to rates of soluble carbohydrate accumulation in the tissue. On TLC, the enzyme products exhibited a marked resemblance to the complement of native fructan in the source tissue. The data demonstrate that the species specificity of the fructan complement can be explained by the properties of synthetic enzyme(s) alone, and is not dependent upon structural features of intact cells or tissues. The established view of fructan synthesis holds that the polymerizing enzyme, fructan: fructan fructosyl transferase (FFT), is inhibited by sucrose. The enzymes from all five grass species manufactured fructans of DP ≤ 3 in the presence of high concentrations of sucrose (above 200 mol m−3). Hence the properties of these grass fructan polymerases differ from those of FFT. Copyright © 1993, Wiley Blackwell. All rights reserved